On Mon, Sep 19, 2016 at 10:58:56PM +0200, Olaf Schreck wrote:
> > Check ip neigh output. Does the entry for your default gateway go
> > STALE after those 20 minutes?
> Yes, exactly:
> # ip -6 nei
> fe80::1 dev eth0 lladdr 0c:86:10:ed:31:ca router STALE

So it is the neighbor table entry going stale. Does your system retry
neighbor solicitation (see tcpdump) or does it sit quietly with a
STALE entry? I had this behavior in similiarly styled hosting networks
a few years ago, and since it has fixed itself I suspect a kernel
issue that was fixed since then. I do use more recent kernels than
Debian stable though.

> > Do you really need to meddle with the fe80::1 route?
> I had no plans to do so.  I just learned during debugging that this would 
> reestablish IPv6 connectivity without reboot.

It would be interesting to see whether this causes your system to do a
neighbor solicitation for the default gateway.

> > Do you really
> > need an explicit route for fe80::1%eth0? Will it work without?
> My hosters docs (Hetzner) recommend that.  They don't specify an IPv6 
> default gateway.

They're unfortunately not very clueful with regard to IPv6. I haven't
been on their network for years though.

> > or is it
> > really necessary to remove the default route and to re-add it?
> yep, connectivity is back


> > No need, an fe80::/64 IP address is only valid when an interface is
> > added:
> [...]
> > Is the other interface connected? eth1 should not play a role here at
> > all.
> I had no plans to fiddle with link-local addresses, and of course eth1 
> settings should not matter.  I just disabled IPv6 on eth1 for debugging, 
> and suddenly IPv6 worked >20min.  Maybe coincidence rather than causality. 

Check whether your system re-solicits for the default gateway with
eth1 down and/or up. If its re-solicitation behavior on eth0 differs
depending on eth1's state, we have a clear system software issue here.

If we're not talking about jessie but something more recent,
systemd-networkd might play a role. systemd-networkd has recently
started to take over IPv6 mechanics, and does so in a quite broken way.

> I'd like to learn what's going on here,

Me too

> thanks for your comments.

You're welcome


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